Electrical piano



Oct. 29, 1940.

H. F. RIEQHERS ELECTRICAL PIANO Filed March 20, 1939 AMPL [FIE/Q INVENTOR fkrfier I 1? Leo/wits 71/ ATTORNEY Registered Oct. 29, 1940 UNITED STATES PATENT OFFICE signor to. The Ansley Corporation,

New York,

Application March 20, 1939, Serial No. 282,817

9 Claims.

The present invention concerns an electrical balance system and in particular a novel balance system in an electrical musical instrument.

One object of the invention is to provide a balance system in an electrical amplifier system in which undesired signals are substantially eliminated and desired signals are unattenuated.

Another object is to'eliminate undesired signals from more than one source while preserving unattenuated a plurality of desired signals.

A particular object is to eliminate undesired disturbing signals in an electrical musical instrument without change in any desired signals.

Another particular object is to provide an electrical piano in which hum and other disturbing signals due to stray fields and power supply current are substantially eliminated by means of a balanced amplifier while the sound signal picked up from the strings electro-statically are unafiected.

These and other objects will be evident from the detailed description of the invention which is given in connection with the various figures of the drawing.

The present invention is to be shown in connection with an electrical piano in which vibrations of the strings are picked up electro-statically, amplified and applied to a loud speaker. A bias voltage is applied between the strings and pick-up buttons arranged to have predetermined capacities to the strings. The sound signals to be amplified and reproduced appear as modulations of the voltage between strings and pickup buttons due to the vibration of the capacity between them.

In this type of system there are two sources of disturbing signals. Alternating current fields exist due to power supply equipment and various wires and circuits carrying alternating current.

These fields are picked up by the sound signal pick-up buttons and amplified producing undesired hum and other extraneous noises. Also hum and other disturbing voltages in the bias voltage source are applied to the amplifier input since these voltages appear in parallel with the bias on the pick-up buttons. In the past electrical field disturbances have been made less disturbing by elaborate electro-static shielding of the strings and pick-up buttons. This is not entirely efiective since the apparatus is large and formed of many parts rendering complete shielding diflicult and complicated. Disturbances in the bias voltage source have been reduced by deriving a counteracting alternating current voltage from the amplifier heater supply. This attempt to balance requires careful adjustment and is not satisfactory due to the fact that the wave form of the bias supply hum and the heater voltage is usually considerably different and of indeterminant phase relation.

The present invention consists in dividing the pick-up buttons into two groups and amplifying signals from each group thru two channels the outputs of which are connected in opposition. Disturbing electro-static field voltages appear equal and in phase at the inputs of the two channels and are cancelled in the output. At the same time disturbing voltages from the bias supply are applied equally and in phase to the inputs of the two channels and cancel in the output. In the case of both sources of disturbing signals equivalence in wave-form, phase and amplitude are assured and cancellation is substantially complete for all components. The desired signals have different frequencies which causes no cancellation in the output. Since it may not be possible or convenient to provide accurately symmetrical pick-up systems, a fine balance adjustment may be provided.

In the drawing:

Fig. 1 shows a diagram of an electrical piano embodying the present invention.

Fig. 1a shows details of the bridge and string arrangement of a piano embodying one form of the present invention.

Fig. 2 shows in detail the balanced amplifier circuit.

Fig. 3 shows an alternative balance connection.

Fig. 4 shows another alternative balance connection.

Fig. 1 shows a piano key-board having a plurality of keys I. Each key I actuates a striker 4 through a linkage 2-3 for striking strings 5 and 1. Along the bridges 40, 4| and 42 (Fig. 1a) under strings 5 and 1 are arranged electrostatic pick-up buttons 6 and 8. There is one button 6 or 8 for each string or group of strings comprising a single note of the piano. Buttons 6 associated with one group of strings 5 are all connected in parallel and thru lead 9 to one side of the double input amplifier It. The remaining buttons 8 associated with strings I are connected in parallel and thru lead II to the other side of the input to amplifier Hl. Strings 5 and 1 are grounded thru lead H to ground G which also forms the center point of the input to amplifier l0. Amplifier i0 is arranged so that equal inphase signals applied to the two sides of its in-. put are cancelled in its output. The desired signals due to the vibrations of strings 5 and 1 will be passed without attenuation since they are of differing frequencies and random phases. These desired signals are amplified by amplifier l0 and applied to loud speaker I3 by means of leads l4 and 15. In the piano as shown in Fig. la there are two groups of strings 5 and 1. Strings l are the treble strings and are associated with the two part treble bridge Buttons 8 are mounted on treble bridges 40 and H. The bass strings 5 are associated with bass bridge 42 and energized from power supply 3|.

buttons 6 mounted on brid e 42. It has been found that, substantially equal hum voltages are picked up by the buttons on the two bridges although there are more buttons on the treble bridge. A good balance of hum is obtained by connecting buttons 8 on treble bridge 40-" to one side of the balanced amplifier input and buttons 6 on bass bridge 42 to the other side of the amplifier input. This equality of hum-voltages is due to the fact that the hum field is somewhat greater near the bass bridge. If the hum field were homogeneous equal numbers of buttons should be connected to the two sides of the amlifier input. D Fig. 2 shows details of the pick-up circuit, bias voltage application and amplifier balance circuit. Only a few piano strings and pick-up buttons are shown in order to simplify the representation. Hammers 4 are adapted to strike a. predetermined string or group of strings 5 and I. Under each string or group of strings 5 and 1 are located electro-static pick-up buttons 6 'and 8. Strings 5 and l are connected to ground G. One group of buttons 6 is connected together and thru lead 3 and coupling condenser Hi to control grid 26 of a thermionic vacuum tube IS. The other group of pick-up buttons 6 is connected together and thru lead ii and coupling condenser ii to control grid 23 of a second thermionic vacuum tube IS. A bias voltage is supplied between strings 5 and 1 and buttons 6 and 8 from a power supply 3| thru coupling resistors Hand 36 respectively. Tube l8 includes a cathode 2|, control grid 26 and plate 22. Tube l9 similarly includes a cathode 24, control grid 23, and plate 25. Grids 26 and 23 receive signals asdescribed and are connected across a grid resistor 26 the return point 21 of which is adjustable for balance adjustment purposes. Cathodes 2| and 24 are connected thru a common cathode bias resistor 28 to ground G. Plates 22 is connected to one end 34 of the primary of a push-pull output transformer 34-35-36 while plate 25 is connected to the other end 35 of the push-pull primary. Voltages induced in secondary 36 are applied to an amplifier 31 the'output of which feeds speaker l3. The center of transformer primary 34-35 is also The negative side of power supply 3| is connected to ground G thru lead 34. A further balancing adjustment is provided by the adjustable condenser 38 connected between strings 5 and pick-up buttons 6. Pick-up buttons 6 may comprise a group associated with the piano strings 5 covering one end of the key-board while buttons 6 are associated with the remaining strings I of the piano. It is convenient to arrange these groups to be approximately equal and if necessary to obtain exact equality by means of condenser 38.

In operation various combinations of strings 6 and I are struck by means of hammers 4. The vibrations of strings 5 and 1 cause variations of the capacity of buttons 6 and 8 to grounded strings. This varying capacity varies the voltage across buttons 6 and 8 to ground G, the average value of whichv voltage is maintained by power supply 3| thru the high resistances 29 and 36. These varying voltages represent desired sound signals. Sound signals from buttons 6 are amplified by tube l8 and sound signals from buttons 8 are amplified by tube IS. The amplified sound signals appear across primary 34-35 and hence across secondary 36. These sound signals across secondary 36 are amplified by amplifier 31 and fed to loud speaker l3 which reproduces them as greatly amplified piano sounds. There is no cancellation or modification of sound signals in amplifying them thus in two groups and in the push-pull output of tubes l8 and i6 since they represent signals of different frequencies and random phases.

However, stray fields due to leakage from power supply 3| and various alternating current circuits will be picked up by buttons 6 and 6. These stray voltages will be in phase on button groups 6 and 8 and may be made to be substantially equal by making groups 6 and 8 substantially equal in capacity to ground. Small inequalities in capacity of .the button groups or of the fields at the group locations may be compensated by adjustable condenser 38. These equal in-phase stray voltages are applied to grids 26 and 23 of tubes i8 and I9 and are thus amplified and fed to separate sections of transformer primary 34-35. Primary 34-35 is in push-pull connection so that in-phase voltages applied to grids 26 and 23 result in equal but out-of-phase voltages in secondary 36 and hence are cancelled. Similarly, power supply 3| will have a hum or disturbing voltage component across its output which is applied to grids 20 and 23 equally and in-phase thru resistors 29 and 36 and blocking or coupling condensers i6 and I7 respectively. These inphase disturbing voltages will cancel when they are amplified and reach secondary 36 as described above. Small differences in tubes l8 and I9 and circuit constants may be compensated by adjustment of return point 27 of grid return resistor 26. Thus, the system operates to amplify without modification the desired piano string sound signals, and cancels both stray electro-static fields picked up by the buttons and stray disturbances originating in power supply 3|. The system is symmetrical providing substantially equal pickup of both desired and undesired signals on both sides of the balancing system. Both desired and undesired signals are opposed in the output of the preliminary amplifier but only the undesired signals cancel due to the random frequency and.

phase characteristics of the desired signals.

While the system shown in Fig. 2 is a preferred form many modifications of the system are possible. Fig. 3 shows one such modification. In Fig.

3 instead of dividing the pick-up buttons into 4 two divided groups alternate buttons are connected together. Alternate buttons 6 are connected to alternate buttons 6 and by lead 3 to one side of the input to the balanced amplifier it. The remaining alternate buttons 6' and 6 are connected together and thru lead I to the other side of the input to amplifier iii. The output of amplifier I0 is connected to loud speaker i3. It will be seen that this method of dividing the buttons results in a. system in which stray fields and power supply hum are balanced while desired signals are amplified without modification.

Fig. 4 shows still another modification in which buttons 6 are connected together and to a stray field pick-up plate 40 and thru lead 9 to one side of the input to balancing amplifier l0. Buttons 8 are connected together, to a stray field compensating pick-up plate 39 and then thru lead II to the other side of the input to balancing amplifier Ill. The output of amplifier |6 feeds loud speaker i3. Plate 40 is located near buttons 6 and is of a size to substantially equal the capacity and stray field pick-up of buttons 6 while' pick-up of buttons 6. The stray field pick-ups thus equalized apply equal in-phase disturbing voltages to the two sides of the input of amplifier l0 and these voltages are cancelled within the amplifier by push-pull connection.

It will be evident that other modifications are possible as, for instance, condensers I6 and I! may feed opposite sides of a push-pull input transformer the secondary of which feeds a single-ended amplifier input or cancellation may be obtained by using other phase reversing means in the amplifier.

While the pickup buttons 6 and 8 have been described as electro-static they may be of other types such as magnetic pick-up coils, crystals or any suitable pick-up device. The hum balancing system will be as effective with these other types of pick-up as it is with the electro-static type.

While one system and various modifications have been shown for carrying out the present invention, other modifications are possible within the spirit and scope of the invention as set forth in the appended claims.

-What is claimed is:

1. In an electrical musical instrument embodying electro-static pick-up of signals from vibrated strings, the combination of, a plurality of electrostatic elements, a source of imperfectly filtered bias voltage, circuits connecting said elements into two substantially equal capacity groups, circuits for applying a substantially equal bias to each of said groups from said source, and a balanced amplifier for substantially cancelling equal voltages appearing across said two groups.

2. In a musical instrument embodying a loud speaker reproducing amplified signals derived from electro-static pick-up of vibrations of a plurality of strings, the combination of, two'groups of electro-static pick-up elements for' generating desired signals from said vibrating strings, an adjustable condenser connected across one of said groups for equalizing the capacities of said two groups, an amplifier for amplifying signals from one of said groups, an additional amplifier for amplifying signals from the other of said groups, a control for equalizing the gains ,of said two amplifiers, a transformer for subtracting the signals from one of said amplifiers from the signals from the other of said amplifiers for substantially cancelling equal in-phase disturbing signals picked up by said two groups of pick-up elements.

3. In an electro-acoustical sound system, the combination of, two groups of desired-signal generating means, independent signal pick-up means subjected to a common interference field and coupled to said groups of signal generating means, means for.c0mbining signals picked up by said pick-up means in opposed phase to produce substantially interference free desired signals, and electro-acoustical means for reproducing said combined desired signals.

4. In an electro-mechanical musical instrument embodying electro-static pick-up of useful signals from vibrating strings, the combination of, a plurality of electro-static pick-up elements subjected to capacity variations to said strings and an interfering field, circuits for connecting said elements in two groups, a source of imperfectly filtered bias voltage, independent circuits for applying said bias tosaid two groups of elements, a balance circuit for adding signals derived from form a first group, circuits for connecting together the remaining elements to form a second group, a balance circuit for combining the signals from the two groups in phase opposition to-substantially cancel undesired signals due to said interfering fields, and a utilization means to receive in unattenuated form the signals due to said strings.

6. In an electro-mechanical musical instrument embodying electro-static pick-up of useful signals from vibrating members, the combination of, a plurality of electro-static pick-up elements associated with said members and subjected to undesired interfering fields, circuits for connecting said elements into a first group and a second group, a pick-up located near said first group and connected to said second group for picking up signals from said undesired fields, a pick-up located near said' second group and connected to said firstgroup for picking up signals from said undesired fields, circuits from combining the signals from said two groups and their connected pick-ups in phase opposition, and means for utilizing the resultant of combined signals.

7. In an electro-acoustical system, the combination of, two groups of signal pick-up elements subjected to substantially equal undesired signals of equal phase and independent desired signals of random phase, a circuit for combining the signals from both groups in phase opposition to produce unattenuated desired signals substantially free from undesired signals, and means for applying said unattenuated desired signals to a utilization means.

8. In an electro-acoustical system for producing substantially interference-free signals the combination of, two amplifier input points, circuits embodied in said amplifier for substantially cancelling equal inphase voltages-applied to said combination of, a plurality of signal generating ference free useful signals, and means for reproducing said useful signals.

HERBERT F. RIECHERS. 

